During printing onto sheets, for example paper sheets, in particular during output of the printed sheets, the latter may be incorrectly stacked. Sheets that are not treated further, for example guiding or stiffening of the sheets, and are stacked out of the printing press on a stacking tablet, for example, can be kinked due to the inadequate rigidity of the paper sheets and thus will come to lie on the tray incorrectly, i.e., will be displaced or crossed over.
The above-described problem of kinking occurs in particular with long sheets, which if possible should not contact the stacking tray before they have almost fully left the printing press. The kinking results because the sheet cannot move freely on the stacking tray because part of it is still inside the printing press, and consequently a backlog may result.
In general, this incorrect stacking of sheets is prevented by deformation of the sheets in a particular manner, so that the sheet is stiffened as a result of this deformation. As a result it is possible for the sheet after leaving the printing press to remain free of kinks and for it to be guided out of the press with a substantially constant alignment, and after completely leaving the press to be stacked on the stacking tray in a controlled manner.
A person skilled in the art will be familiar with systems, so-called stiffeners, which bring about a stiffening of printed sheets to be output. Transport roll pairs, located in one plane and arranged with lateral displacement, are for example provided for this, having for example complimentary-shaped ball-shaped and spindle-shaped rolls. This design of the rolls results in the sheets to be output being deformed at different points in the region of the transport roll pairs, i.e., locally deflected out of a transport plane in which the sheet was previously transported so that a type of wave surface is formed. The sheet can be stiffened as a result and be stacked on the stacking tray after leaving the printing press without kinking. With this known system, the transport roll pair assumes the functions of both transport and stiffening.
A drawback of stiffening of sheets in this way is the formation of artifacts, such as pressure points and/or a smudging of the printed content as a result of the tightly-limited deformation in the region of the transport rolls. With long sheets or very light sheets in particular, a substantial deformation of the sheet is necessary to obtain the required stiffness, as a result of which the sheet does not kink before it is laid on the stacking tray. With a device as described above, an increase in deformation would lead to an increase in the described artifacts.
Starting from the above-described situation, it is therefore an object of the present invention to provide a controlled stacking of a sheet such that there is no deterioration of the printed content of a previously applied print medium and no damage in any way to a sheet due to its being placed on a stacking tray, for example through pressure points on the sheet.